In a MEMS gyroscope drive loop, an electrical current (e.g., current Igyro) is generally created from mechanical displacement of the gyroscope and converted to electrical voltage for a phase-lock loop (PLL) input signal. This current Igyro, however, can have a wide range covering many orders of magnitude. For example, in an approach, the range expands four orders of magnitude, such as from 0.2 nA to 2 μA. To convert this wide current range to a reasonable voltage level, a corresponding wide range of variable resistance (e.g., a resistor Rimp) is used so that the large resistance value at the upper range can correspond to the low current value amplification. Because of the large resistance requirement and thus large die area, resistor Rimp is typically implemented “off-chip,” i.e., outside of the die/semiconductor chip embodying the MEMS structure, and on a board level. Further, resistor Rimp is generally required to be adjustable to maintain sufficient electrical voltage level for various currents Igyro to lock the PLL, to limit the voltage level, and to prevent the device from being overstressed. Adjusting resistor Rimp for appropriate amplification is commonly done manually by human beings, which is not suitable for mass production.
In another approach, to achieve automatic control, an automatic level control (e.g., ALC) unit is designed utilizing an NFET with proper control at the gate to provide resistance in place of the gyro resistor Rimp. The unit, however, uses two stage of amplification to cover the four orders of magnitude difference of the Igyro. The unit also uses sophisticated circuitry with amplifiers and diodes, all of which is prone to errors and large power consumption in the μA range, making the unit inefficient.
Like reference symbols in the various drawings indicate like elements.